In the past solder mask has been applied to a printed circuit board substrate by a variety of wet and dry techniques including screen printing (wet) and lamination (dry film).
The screen printing method provides for selective deposition of a liquid material onto a circuit board substrate. When a small number of printed circuits of a particular design is to be made, hand screen printing may be used. When a large number of printed circuits of the same design is to be made, machine screen printing of the desired circuit is used.
In what may be termed conventional screen printing, the liquid material is selectively deposited on the substrate and then the solvent in which the screen printable material is dissolved is removed by drying in an infrared oven. Although conventional screen printable material is relatively inexpensive, the process of infrared drying is costly and cumbersome because it requires specialized equipment for off line drying and additional manufacturing space and personnel.
Additonally, in the past solder mask has been applied as a dry film using lamination processes. Generally, when a dry film solder mask is used it is applied to the substrate via vacuum techniques. Thereafter, the board is exposed to ultraviolet light and the film is cured where exposed. After the selective exposure, the board is developed in a multitude of solvents including water, and the unexposed film is removed. The board is then dried by heat. Thereafter the board is postexposed in ultraviolet light. A problem with the dry film method is in its lack of automation. At various times during the dry film operation long cycle times are experienced e.g. the boards generally require a lengthy stabilization time between exposure to ultraviolet light and further processing. Additionally, in dry film processes a cover sheet is used which is cumbersome and time consuming to remove. The dry film technique also has the drawback of limiting the thickness of the solder mask to the preselected thickness of the film.
In addition to the conventional method of applying a liquid screen printable solder mask discussed herein, another method of applying a liquid solder mask has been suggested. In this method liquid polymer which is directly curable by ultraviolet radiation is coated, preferably by roller coating, onto a printed circuit board substrate. In this method a phototool having a solder mask design is placed in close relationship to the liquid coating and light is applied through the phototool. After the selective exposure, the unexposed liquid polymer is removed, thereby exposing metal areas which can be coated with solder.
Although in the past several different materials have been used as a solder mask, because of the physical and chemical properties necessary for an effective solder mask, it has been viewed as difficult, if not impossible, to find a solder mask which is directly curable by ultraviolet radiation and which can be coated by means of a roller coater onto a circuit board substrate. The inventors herein have discovered a new material which is curable by ultraviolet radiation and has the physical properties, electrical properties and solderability necessary to allow it to be used as a solder mask. Additionally, this new material has a unique combination of physical and chemical properties which allow it to be applied via roller coating techniques, as well as by conventional screen printing methods. Further, since this new material is directly curable by ultraviolet light it has the advantage that it is be applied to the printed circuit board substrate as a solventless liquid thus eliminating the costly and cumbersome step of infrared drying which is necessary in conventional screen printing methods. Moreover, since the new material is applied as a liquid or a hot melt the vacuum lamination process and other steps used in dry film techniques are avoided. The solder mask of the present invention has the additional advantage that the user is able to adjust the thickness of the solder mask material to his individual and varying needs.